A head mounted display (“HMD”) is a display device worn on or about the head. HMDs usually incorporate some sort of near-to-eye optical system to form a virtual image located somewhere in front of the viewer. Single eye displays are referred to as monocular HMDs while dual eye displays are referred to as binocular HMDs. Occlusion HMDs, also called immersion HMDs, project a virtual image over a black background (the projection optics are not see-through). See-through HMDs also project a virtual image, but they are at the same time transparent (or semi-transparent) and the projection optics are called combiner optics, since they combine the virtual image over the reality. Such optics may be more complex than optics used in immersion HMDs. Augmented reality is one aspect of see-through HMDs, where the virtual image is superimposed to the reality.
HMDs have numerous practical and leisure applications. Historically, the first applications were found in aerospace applications, which permit a pilot to see vital flight control information without taking their eye off the flight path (these are referred to as Helmet Mounted Displays and are often used for rotary wing aircrafts). Heads Up Displays (“HUDs”) are usually used in non rotary wing aircrafts such as planes and jet fighters, where the combiner is located on the windshield rather than on the helmet. HUDs are also used in automobiles, where the optical combiner can be integrated in the windshield or close to the windshield. Public safety applications include tactical displays of maps and thermal imaging. Other application fields include video games, transportation, and telecommunications. There is certain to be newfound practical and leisure applications as the technology evolves; however, many of these applications are limited due to the size, weight, field of view, and efficiency of conventional optical systems used to implement existing HMDs.